blue layout of welcome page - version 5.4
green and black layout of welcome page - version 5.5
in case of CQ5.5 after login in the welcome page on the bottom there is detailed info which version you have
As soon as you login to CQ5 , you will be in http://localhost:4502/libs/cq/core/content/welcome.html page.
On the Right hand side top, you will have DAY Logo , onclick of which you will get to know About your CQ5.